Global navigation satellite systems (GNSS) are the backbones of positioning and navigation solutions. It has been estimated that the total number of GNSS receivers in the markets has exceeded 4 billion units by the end of 2016. A GNSS receiver (e.g. in the form of a chipset) can be found in every smartphone, smartwatch, new car, high-end drone etc. The volumes are still rapidly growing due to the GNSS integration into Internet of Things (IoT) devices.
It is commonly known that a standalone GNSS receiver does not work satisfactorily in urban areas and it also has certain fundamental bottlenecks in its performance that make it non-ideal e.g. for mass market devices and their use cases. GNSS was originally aimed for outdoor (and continuous signal reception) use only, hence the GNSS signals and the data link from the satellites to the receiver were not designed for weak signal conditions nor to the fastest possible time-to-first-fix. Also, the fact that the satellites are far in the space (at an altitude of 20,000 km) and solar-powered means that no engineering effort will be enough to overcome the physical limitations related to limited transmission power and to the radio propagation loss. Solutions to improve the performance of GNSS receivers need to found on other technologies and radios.
Cellular operators and mobile phone manufactures started to develop GNSS assistance data services roughly 20 years ago, to find a solution for the mentioned GNSS performance gaps. It was discovered that the ranging signal transmitted from the satellites was still strong enough in urban environments to be received with the novel high-sensitivity GNSS receivers, but the navigation data interleaved with the ranging signals become too noisy and erroneous for successful demodulation. (Navigation data contains the satellite orbit and clock parameters among other constellation status info, which are essential for position calculation.) Hence, a solution capable of capturing the satellite navigation data and transmitting that data via an alternative route to the receivers would drastically improve the performance and make GNSS acceptable even for emergency call positioning. As a result, assisted-GPS (later assisted-GNSS, A-GNSS) technology was created, standardized and adopted for commercial use. Today, all the GNSS receivers in the smartphones are A-GNSS receivers i.e. inherently combining GNSS and terrestrial systems into one positioning technology.
The GNSS family consists of several satellite constellations. The first and most widely used is the system developed and operated by the US Air Forces i.e. Naystar GPS, in short: GPS (global positioning system). GPS has been in operational use since 1980's. The Russian counterpart from the Cold War era is GLONASS, which has now been modernized and offering performance equal to GPS. China is currently building up their own global system called Beidou and the EU is ramping up Galileo. In addition to these four global satellite constellations, there exist also regional augmentation systems (satellite based augmentation systems, SBAS) such as the Japanese quasi-zenith satellite system (QZSS) and multi-functional satellite augmentation system (MSAS), the U.S. wide area augmentation system (WAAS), the European geostationary navigation overlay service (EGNOS), the Indian GPS aided geo augmented navigation (GAGAN) system and the Russian GLONASS system for differential correction and monitoring (SDCM).
GNSS constellations offer open service (OS) signals for the civilian and “unauthorized use”, and regulated/military signals for the authority and military use, latter of which typically require a specific receiver or encryption keys to use these signals for positioning (even for reception). On the contrary, the structure and format of the OS signals are publicly known, as the interface control documents (ICD) describing the signals and data transmitted by the satellites are freely available. Also, the OS GNSS receivers are commercially available as modules, development kits etc. so it is relatively easy to get access to the GNSS signals and data, even to replicate the signals with perfect receiver compatibility. This “easiness” and openness has led to the development of numerous malicious devices which can be used to “spoof” the GNSS receivers in various ways: either to make them report false position and/or time, or even totally jam/block the performance. None of the existing GNSS systems have any means to authenticate the signals or data the satellites transmit, and hence efficiently avoid spoofing. The lack of signal/service authentication is a very serious risk for the location based services that use GNSS to validate the location of a device or a user e.g. for charging/transactions (road tolls, parking etc.). Especially, for the smartphone use cases this has been seen as one of the major problems.